Access systems may aim to efficiently and cost-effectively generate and communicate a low-frequency magnetic field with a large range. At the same time, they must comply with the radio approval regulations and the EMC guidelines of the automobile manufacturers.
A response of an immobilizer transponder is received, e.g., with a very small coupling factor to the antennas which beforehand emitted a comparatively high (strong) carrier signal (with modulated data) and thus supplied the transponder (TRANS) with energy. The transponder in this case is, e.g., of the “Charge and Talk” type, that is to say that it is firstly charged with energy from a low-frequency magnetic field, acquires data from this field (by demodulation), calculates a response and sends the response back using the stored energy.
Keyless vehicle access and start systems, such as the passive start entry (PASE) system, for example, may be automatic systems which unlock a vehicle without active use of an automobile key and start it just by actuation of the start button. This is made possible by an electronic key carried by the driver of the vehicle. The vehicle periodically emits, via at least one antenna situated on the vehicle, an enquiry signal at an LF frequency (LF stands for “Low Frequency” with frequencies between 20 kHz and 200 kHz, for example), said signal being coded by means of a first coding table. The system thereupon goes into a receiving mode in the UHF range (UHF stands for ““Ultra High Frequency” with frequencies in the three-digit MHz range, for example) and waits for confirmation. If a key is in range, it receives the LF signal, decodes it and re-emits it as a UHF signal with a new coding using a second coding table. The UHF signal is decoded in the vehicle. Since the vehicle knows both coding tables, it can compare its own original emission with the signal just received and grant access in the case of correspondence. If there is no correct response within a defined time, nothing happens and the system switches to standby again. The engine starting process, in some examples, substantially corresponds to that of access control, except that here an engine start button would be actuated.
There are fault situations in which the engine starting process does not proceed functionally, e.g., in the case of interference with the UHF signal or in the case of an absent or discharged battery in the key. The engine start can then be enabled by means of the immobilizer function contained in the PASE system. On the part of the vehicle, an enquiry signal at an LF frequency, said signal being coded by means of a first coding table, is emitted via at least one antenna situated on the vehicle. The system thereupon goes into a receiving mode for receiving the response of the key in the LF frequency range. The key contains a transponder (TRANS) operated without a battery because it obtains its energy for supply from the low-frequency magnetic field and stores it, and said transponder, in the case of the “Charge and Talk” type, sends its response back by means of the low-frequency magnetic field using the stored energy, that is to say dispenses with the use of the UHF transmission.
In this case, an inductive antenna is predominantly used as an antenna for emitting the LF signal, said inductive antenna being embodied for example as a ferrite core provided with a winding (also known as magnetic antenna or ferrite antenna). In this case, the inductance of the inductive antenna is often operated together with a capacitor in a resonant circuit. The energy consumption of such a resonant circuit is usually kept low by a quality factor that is as high as possible and by an exact frequency tuning, in order to minimize the total current consumption of the access and start system. A low current consumption is also desirable, for example, because otherwise the vehicle battery would be rapidly discharged in the event of the vehicle being parked for a relatively long time. However, a high quality factor restricts the transmission data rate and an exact tuning in conjunction with a high quality factor requires some complexity. Therefore, conventional arrangements often constitute an unsatisfactory compromise between data rate, complexity and energy consumption.
Therefore, quasi-resonant resonant circuit drivers are known, with which a high quality factor (and thus a low current consumption) can be achieved in conjunction with a sufficiently high data rate. These driver circuits may have the disadvantage that not necessarily generally all of them comply with the radio approval regulations. The radio approval regulations are intended to ensure that other radio services (e.g., broadcasting (radio and television), mobile radio services (police and security services) or cellular phones) are not adversely affected in their operation. A further disadvantage of such quasi-resonant driver circuits may include failure to comply with the guidelines of the automobile manufacturers with regard to electromagnetic compatibility (EMC).
Furthermore, quasi-resonant resonant circuit drivers are known, which are improved to the effect that not only do they enable a low current consumption and low interference signal emission in conjunction with low circuitry and adjustment complexity, but they additionally also enable compliance with radio approval regulations.
DE 10 2013 220 596 on Oct. 11, 2013 relates to a quasi-resonant resonant circuit driver. DE 10 2014 222 603 on Nov. 5, 2014 relates to suppression of common-mode emission by a circuit arrangement. DE 10 2014 220 406 on Oct. 8, 2014 describes a quasi-resonant resonant circuit driver, wherein FIG. 13 thereof shows a driver channel with source terminals of transistors that are connected to one another back-to-back. DE19541855A1 relates to a method for reducing the reception quality factor and the resonant frequency of the antenna resonant circuit.